A current commercial process for the preparation of aromatic polycarbonates is the phase boundary condensation process. According to this process an aqueous bis-phenolate solution is reacted in phosgene with a suitable solvent for the polycarbonate, optionally in the presence of chain stoppers and with further addition of aqueous alkali metal hydroxide solution.
Solvents recommended for this process are preferably chlorinated hydrocarbons such as, for example, methylene chloride, chloroform, 1,2-dichlorethane and chlorobenzene. While, the use of methylene chloride or 1,2-dichlorethane alone or mixed with other solvents, including, for example, chlorobenzene, presents no special problems, the use of chlorobenzene alone entails great difficulties. These difficulties result from the extremely low solubility of the polycarbonates in chlorobenzene at temperatures below 80.degree. C. Thus, for example, 10% strength polycarbonate solutions (bisphenol A type) in chlorobenzene are stable only at temperatures in excess of about 30.degree. C. and 20% strength solutions are only stable at above about 70.degree. C. At the customary reaction temperatures of 25.degree.-30.degree. C., only about 8 g of polycarbonate will dissolve in 100 g of chlorobenzene.
Therefore, if it is desired to use chlorobenzene as the sole solvent in the synthesis of polycarbonate by the phase boundary process, it is necessary to maintain reaction temperatures of or in excess of 70.degree. C.
The use of chlorobenzene is of great interest for various reasons including the following:
1. better chemical stability, in comparison to the aliphatic chlorinated hydrocarbons, towards the alkaline reaction solution, and very good heat stability when isolating the polycarbonates in evaporation extruders at temperatures between 300.degree. and 350.degree. C., so that light colored polycarbonates are obtained; PA1 2. a simpler and more reliable form of recovery from the process effluents, due to the substantially lower solubility and volatility of chlorobenzene resulting in improved ecological and economical performance and; PA1 3. less corrosion when recovering the solvent by distillation in the presence of water.
Though the reasons mentioned suggest the industrial use of chlorobenzene in the preparation of polycarbonates, industrial use has hitherto been prevented by the difficulties which occur during phosgenation at elevated temperatures. It becomes difficult to reproducibly conduct the condensation reaction. Saponification reactions occur with both the phosgene and the oligomeric intermediate products having chlorocarbonic acid ester end groups. The reaction requires increased amounts of phosgene compared to lower temperature phosgenation and it becomes impossible to control the molecular weight distribution of the finished polymer to the degree of precision desired. It is believed that this is because at an elevated reaction temperature the saponification reaction of phosgene or of the chlorocarbonic acid ester groups is greatly favored compared to the propagation reaction which leads to the formation of the carbonate polymer. A consequence is that the bisphenols employed are not incorporated quantitatively into the product.
It is clear that an increased consumption of phosgene, incomplete incorporation of the bisphenols employed and molecular weights which do not have the desired values for the particular grade of product, thus producing waste material - are severe economic and technical disadvantages.